Process and apparatus for the continuous preparation of polymers
The invention relates to a process and an apparatus for the continuous preparation of polymers in which at least two reaction partners (starting materials) are conducted through a micromixer and combined and mixed with one another.
The reaction partners are, for example, one or more liquid or dissolved monomers and one or more initiators.
DE-A 19 41 266 discloses a process for carrying out chemical reactions between gaseous and/or liquid reaction partners, in which at least two reaction partners are divided into spatially separate microstreams by a multitude of microchannels assigned to each of them, which microstreams then exit into a shared mixing and reaction compartment. The microstreams exit, in the shape of fluid lamellae of the starting materials, from the microchannels into the mixing/reaction compartment at equal flow rates for each starting material. Each fluid lamella of a starting material is conducted into the mixing and reaction compartment immediately adjacent to a fluid lamella of another starting material, the adjacent fluid lamellae mixing with one another by diffusion and/or turbulence.
For the mixing, use is made of what is termed a microstructure lamella mixture having at least one mixing chamber and an upstream guide component for feeding the fluids to be mixed to the mixing chamber. The guide component can be composed of a plurality of plate-like elements which are layered one above the other and are pierced by channels which run at an incline or transversely to the longitudinal axis of the micromixer. The channels of adjacent elements cross without contact and open out into the mixing chamber. Another constructional possibility of a micromixer is a parallel arrangement of channels. The fluids to be mixed flow from opposite directions into the mixer and exit mixed and perpendicularly thereto into the mixing chamber. The mixer brings the reaction partners intimately into contact with one another, i.e. mixes them well with one another. As is implied by the cited document, the quality of the mixing and the effect of the mixing means on the yield of the desired product is greatly dependent on the ratio of the chemical reaction rates given by the reaction kinetics to the mixing rate. In the case of chemically slow reactions, these generally proceed considerably slower than the mixing. If the chemical reaction rates and the mixing rate are of the same order of magnitude, complex interactions occur between the reaction kinetics and the local, turbulence-specific mixing behavior in the reactor used and in the mixing means which is generally a micromixer. If the chemical reaction rates are considerably faster than the mixing rate, the reaction rates and the yields are essentially determined by the mixing, i.e. by the local time-dependent rate and concentration field of the reaction partners.
In the prior art, it is customary to use a number of mixing means or mixers to carry out fast reactions. In this case, a distinction is made between dynamic mixers, such as agitators, turbines or rotor-stator systems and static mixers such as Kenics mixers, SMV mixers and jet mixers.
In the field of solution polymerization of acrylate-based monomers, with or without additives, batch or semibatch technology is usually used, which cannot ensure uniform product quality, owing to the batchwise preparation of polymers. The changeover from the known batch or semibatch technology to a continuous production of solution polymers is accompanied by problems, to the extent that polymers of this type can become insoluble in the solvent at high molecular weights. In the case of a polymer defined as a solution addition polymer, it is a small, high-molecular-weight fraction of the polymer in the molar mass distribution. This high-molecular-weight fraction can, inter alia, be formed by an initially poor mixing of monomers and initiator, since a local deficiency of initiator can lead to the formation of macromolecules having a very high degree of polymerization, which are known to form, in the case of free-radical polymerization, within a time of less than one second. These high-molecular-weight fractions lead to a considerable broadening of the molar mass distribution up to the formation of bimodal molar mass distributions. This causes unwanted deposits to form in the reactor system. Precipitation of the insoluble molecules out of the solution is known to be favored by solid surfaces, such as reactor walls, internals, etc. In tube reactors, which are frequently equipped with static mixers to intensify the mixing operations and heat transfer, there is a high and thus unfavorable surface/volume ratio. This means that, in comparison with a stirred-tank batch reactor of comparable capacity, a higher probability of deposit formation in the reactor system must be assumed, which in the case of the continuously operated tube reactor can lead to a blockage of the system and excludes long-term operation. The generally low amounts of polymer having high molar masses in the product mixture can be sufficient to block a tube reactor, since the process is operated over very long periods. If in the case of the tube reactor there is, at the start of the reactor during metering, a poor homogenization of the reaction mixture over a system-dependent mixing section, intensive deposit formation can occur, particularly in this area.
With the acrylate-based monomers, these can be, for example, copolymers, as are described in DE-A 40 27 594. These addition copolymers are potentially based on alkyl esters and functionalized alkyl esters of xcex1,xcex2-ethylenically unsaturated carboxylic acids with or without copolymerizable vinyl monomers. A further monomer is, for example, styrene.
EP 0 749 987 A1 discloses a process for continuous anionic polymerization. In this case the monomer system consists of at least one (meth)acrylic acid monomer. The initiator consists of organometallics, as are used for the anionic polymerization. As reactions of this type are very fast reactions which lead to complete conversion within 0.2 to 0.3 seconds, an adiabatic tube reactor having an upstream micromixer was developed for a continuous reaction procedure. The micromixer is a turbulently mixing tangential flow mixer. The residence time in the mixer is approximately 0.05 seconds. The starting materials (monomers, solvent and initiator) are, before being fed into the mixer, cooled to from xe2x88x9214 to xe2x88x9240xc2x0 C. to prevent the reaction from starting in the mixer. The reaction occurs in the tube reactor. Owing to the adiabatic reaction procedure, depending on the monomerinitiator system, a final temperature of from 44xc2x0 C. to 91xc2x0 C. is achieved.
Starting from the above-described prior art, the object of the invention is to provide a process and an apparatus for continuous production of free-radical solution polymers, in which blockage or plugging of the reactor system is substantially avoided and the apparatus can be operated for relatively long periods without interruption.
This object is achieved according to the invention by a process of the type described at the outset in such a manner that the starting materials are, prior to their entry into the micromixer, preheated to the extent that they reach a required reaction temperature after entry into the micromixer, in which they are mixed by diffusion and/or turbulence in such a manner that formation of bimodal molar mass distributions or high-molecular-weight fractions is suppressed and that a polymerization of the monomeric reaction partners takes place in a tube reactor downstream of the micromixer. For this purpose it is necessary that mixing takes place immediately, i.e. that the mixing time is less than the reaction time to form an individual polymer chain. Preferred mixing times are, depending on reaction time, in the range from one second to instantaneous mixing. Typical reaction times are familiar to those skilled in the art and are, depending on reaction type and temperature, in the range from milliseconds to a few seconds. As a result of the preheating, the required reaction temperature can be achieved immediately after entry into the micromixer.
Particular embodiments of the process according to the invention are disclosed in the subclaims.
In a development of the process, the one starting material of acrylate-based monomers having a styrene addition and a solvent is passed through a first heated heat exchanger. Optionally, the one starting material of acrylate-based monomers, but without styrene addition, and with a solvent, flows through a first heated heat exchanger.
In addition, the other starting material of a free-radical initiator and, if appropriate, a solvent is passed through a second heated heat exchanger.
In carrying out the process, the starting material of monomers/solvent and the starting material of initiator/solvent is fed into the micromixer in a mixing ratio of from 1:1 to 10:1, in particular 9:1.
The apparatus for the continuous preparation of polymers, having reservoirs for the reaction partners, metering and control devices, filters, and with or without premixers, features a heated heat exchanger being connected in each case downstream of both the reservoirs for the starting material of monomers and if appropriate solvent and the reservoirs for the starting material of initiator and if appropriate solvent, each of the two heat exchangers being connected via lines to the micromixer and the micromixer being connected to a tube reactor which is connected to a discharge vessel for the solution polymers.
The further development of the apparatus according to the invention can be taken from the features of patent claims 8 to 10.
The process according to the invention (including its embodiments) is distinguished from the known process of EP 0 749 987 A1 by the fact that the known process relates to anionic polymerizations, whereas the process according to the invention relates to free-radical polymerizations. Thus different initiator systems are used. The known process is based in addition on an adiabatic temperature regime of the tube reactor. The novel process can comprise a controlled temperature regime having defined settable temperatures, which is expedient for the reaction procedure of the free-radical polymerization. In the known process, the starting materials are fed to the micromixer at from xe2x88x9213xc2x0 C. to xe2x88x9240xc2x0 C. Heating is carried out by the heat of reaction formed in the subsequent polymerization. In the novel process, the starting material streams are preheated in such a manner that after the mixing, preferably immediately after entry, a starting temperature of, for example, 120xc2x0 C. (depending on the reaction type) is present in the micromixer. Excess heat of reaction which would lead to heating of the reaction mixture can be removed from the system by conventional cooling systems. Owing to the differing starting temperatures in the known process and in the novel process according to the invention, in the known process, the reaction takes place exclusively in the tube reactor which is connected downstream of the micromixer. In the novel process, the reaction can take place as early as in the micromixer. In the known process, the micromixer is described by a tangential-flow mixer which mixes exclusively by turbulence. In the novel process, micromixers which have a lamella structure and mix by diffusion and/or turbulence are preferred.
In the invention, to improve mixing, a micromixer is used. In this, the two starting material streams to be mixed are combined via very fine lamella channels in such a manner that mixing of the starting materials in the micro range is present as early as the meeting of the streams. Owing to the construction, in such a micromixer, extremely small channels are present which lead to an extremely high surface/volume ratio, as a result of which the probability of deposit formation in the mixing system and thus the probability of blockage of the mixer should increase greatly. However, surprisingly, owing to the very good mixing of the starting materials, the formation of a high-molecular-weight fraction can be avoided, so that no insoluble high-molecular-weight fractions are formed in the molar mass distribution and despite the extremely high surface/volume ratio, no deposit formation occurs in the reactor system.